This invention relates to laser-based gravity gradiometers and more particularly to a gravity gradiometer in which the torque caused by gravity gradient on a mass dipole is sensed by a laser force-measurement device.
The prior art believed to be most relevant to the present invention is U.S. Pat. No. 3,786,681, issued to Keihn, on Jan. 22, 1974, and entitled "Electromagnetic Wave Modulation and Measurement System and Method." The preferred embodiment disclosed by the patent includes a ring laser having a plurality of circular polarization modes and a modulator element within the laser cavity which produces differential frequency shifts between the polarization modes in response to application of a torque. One use taught for such a device is a gravity meter in which the force of gravity on a known mass is converted to a torque by being attached to the modulator element by means of a lever arm. Such a device is suggested for use in a borehole for measuring the earth's gravity. As is well known and as is taught in the patent, such a device is also an accelerometer and responds to any acceleration of the measuring instrument. Thus, to measure gravity at location in the borehole, the measuring device must be stationary for a time period long enough to allow the device to stabilize. It would be much more advantageous to have a gravity-measuring device which would allow the required gravity measurements to be taken while the device is moved through the borehole.
In normal gravity meter borehole logging, the parameter of most importance is the difference in gravity at known, closely spaced locations in the borehole, that is, the gravity gradient over short intervals. Gravity gradiometers of various types are well known in the art but have not been used in a borehole due to their large size, sensitivity to motion, and long settling times. One form of such gravity gradiometer involves mass dipole suspended at a 45.degree. angle relative to the direction of the gravity gradient to be measured. It is known that a gravity gradient which is neither parallel nor perpendicular to the axis of the mass dipole will exert a small torque on the mass dipole. Horizontal gradients have been detected by supporting the dipole on a fine filament which allows the dipole to turn slightly in response to a gradient. Other efforts have been made to support the dipole on a bearing, ideally frictionless, which would allow the dipole to rotate in response to the torque with the rotation, and therefore the torque, being sensed by electrostatic sensors. As a result, most of these systems have been either very complicated or fragile and tend to have very long settling times so that their use in the borehole would still require stationary reading, and, therefore, long time periods in which the borehole must be out of use.